In an internal combustion engine, for example, in an automotive engine, a fuel such as gasoline and light gas oil may not be completely combusted under the insufficient warm-up condition at the time of engine startup or the like, and in turn, an unreacted material such as carbon monoxide and hydrocarbon is sometimes contained in the exhaust gas. An exhaust gas purifying catalyst for oxidizing carbon monoxide and the like contained in an exhaust gas and thereby purifying the same usually uses a noble metal as an essential component. However, from the resource standpoint, it is requested to use a non-noble-based exhaust gas purifying catalyst in place of such a noble metal-based catalyst.
As the non-noble metal-based exhaust gas purifying catalyst, a catalyst in which an oxide of non-noble metal such as cobalt oxide (Co3O4) is supported on a support is known, but this non-noble metal-based exhaust gas purifying catalyst does not satisfy the exhaust gas purification activity, and particularly the exhaust gas purification property at a low temperature, and therefore improvement thereof is required.
Also, with respect to the non-noble metal-based exhaust gas purifying catalyst, Patent Document 1 states that an exhaust gas purifying catalyst obtained by mixing zirconium oxide or titanium oxide having supported thereon copper (Cu) and/or cobalt (Co), and a copper-substituted zeolite, can remove nitrogen oxide and carbon monoxide in an exhaust gas. In Patent Document 1, a catalyst obtained by mixing a first component in which copper is supported on zirconia or titania, and a second component that is a copper-substituted zeolite, is described as a specific example.
Furthermore, Patent Document 2 states that an exhaust gas purifying catalyst in which silver (Ag) ion or its oxide and cobalt (Co) ion or its oxide are supported in a mordenite structure has a nitrogen oxide (NOx) conversion property.
In addition, Patent Document 3 states that an exhaust gas catalyst containing a catalyst powder composed of an oxide of a transition metal element selected from the group consisting of manganese, iron, cobalt, nickel, and copper and an oxide of a rare earth element selected from the group consisting of cerium, praseodymium, neodymium, yttrium and scandium, can oxidize an unreacted material such as carbon monoxide and hydrocarbon and can reduce nitrogen oxide. In Patent Document 3, a purifying catalyst obtained by adding an aqueous sodium hydroxide solution to a mixed solution of an aqueous cobalt nitrate solution and an aqueous cerium nitrate solution to form a precipitate, and drying and firing the precipitate, is described as a specific example.